Retention agents are generally used in paper production to flocculate dispersed or emulsified colloidal particles such as filling agents, resin dispersions, fibers, and others. The term high retention in this context means that one obtains a high proportion of the stock added to the headbox on the wire after formation.
Example of retention agents used are so called anionic active, or cationic polyacryl amides, modified polyamide amines, polyethylene imines, polyamines, cationic or amphoteric starch derivatives, as well as inorganic chemicals such as aluminium sulphate, etc.
As it is often advantageous to form paper at a low concentration in the headbox, it is also advantageous if the retention chemicals accelerates the dewatering on the wire and in the press parts. A high dry substance after the press is desired as it reduces the drying costs. Hereby each percent of increased dryness is of very great economical importance.
It is previously known from SE-A-78009040-0 to use alum and a cationic starch in a paper forming process, whereby the alum dose is divided in such a way that half the amount of alum is added to the stock in the machine chest, whereupon cationic starch is added close to the head box, and finally the second half of the dose of alum in added close to the head box as well. The method does not, however, give a satisfactorily result with regard to filler retention.
It is also known that one can combine different retention agents with each other, and thereby to obtain strong additive effects with regard to the retention effects of these. It is further known that some combinations of chemicals give the desired effect of simultaneously good retention and dewatering. Among commersially useful systems of chemicals used, COMPOZIL.sup.%, a combination of cationic active starch and colloidal silica, and HYDROCOL.sup.R, a combination of cationic polyacryl amide and alkali activated montmorillonite clay, can be mentioned.
These systems, however, show a considerably drawback by being economically burdensome on the paper production process.